Leuschner, H.

Matussek, P.

Michaelis, W.

KITopen

August 1969Institut für Angewandte KernphysikRadiative Neutron Capture in Fissionable NucleiW. Michaelis, H. Leusch..ner, P. MatussekKFK 1095Reprint from"NEUTRON CAPTUREG~J\~MMA-RAYSPECTROSCOPyfIINTERNATIONAL ATOMIC ENERGY AGENCYVIENNA, 1969RA DIA TIVE NEUTRON CAPTUREIN FISSIONABLE NUCLEIW. MICHAELIS, H. LEUSCHNER, P. MATUSSEKInstitut für angewandte Kernphysik,Kernforschungszentrum Karlsruhe,Karlsruhe, Federal Republic of GermanyAbstractRADIATIVE NEUTRON CAPTURE IN FISSIONABLE NUCLEI. Because of the great success of the(n,y) method in nuclear sttucture studies, it is desirable to extend such measurernents to fissionable targetnuclei, A suitable spectrometer for mvestigating the high-energy radiative capture spectrum without signi-ficant interference from the fission process has been insta.lled at the Karlsruhe research reactor FR-2. TheInstrument Is briefly discussed , First preliminary results are presented,Studiesof the radiative neutron capture process have revealed de-tailed information on nuclear structure. As yet, however, the applicationof this method has been restricted to cornpound nuclei which are stableagainstnuclear fission. With fissile target material the capture spectrumis masked by the prompt gamma-ray spectrum from fission and - to alesser extent - by the delayed gamma rays arising from the beta decaycf tl:l€ fission pr-educts-; The target nucleus 235 U , for Ins tanc e , has a thermalneutr-on fission cross-section of 578 b while the capture cross-section isonly 101 b , Thus the thermal neutron capture reaction accounts for 14.9%of the neutron absorption in 235U, with the fission process accounting forthe remaining 85.1%. For the total energy released in prompt photon emis-sion from fission values of 7.2 and 8 MeV are given [1]. This energy isdistributed between 9 ± 2 photons per fission. The multiplicity for theradiative capture process is probably less than 5 photons per capture. Thusit is evident that the prompt garnma rays emitted in the fission process pro-vide the dominant structure in the 235U spectrum. As further confirmationof this conclusion, a comparison of the spectra from uranium and plutoniumshows that these spectra are essentially identical; only very minor, andprobably not significant, differences occur [2] .Becauae of the great success of the (n, 'Y) method in nuclear structurestudies, it is desirable to extend these measurements on fi.as.ionabfe riucle i ,Moreover, such an extension will reveal new possibilities for the non-destructive assay of nuclear fue1. Preliminary attempts should be directedto the high-ener-gyspectr-um: sirice at high energies the most favourableintensity distribution between capture gamma rays, prompt radiation fromfission and delayed photons from fission products i s expected. As has beenproposed by one of us [3], the interfering component from fission may besuppressed by detecting the fast fission neutrons in anticoincidence and in471" geometry. Since the average number of neutrons emitted per fission is2.5 or everr 3.0, a detection efficiency of about 70% for the single neutr-onis sufficient to obtain 95% total efficiency. The proposed principle can besketched as follows: The sampIe is suz-r-ounded by a plastic or a liquidscintillator of appropriate thickness. Neutrons scattered in the target are5152 MICHA];Ll~ et ai,photomultiplierdetectorFIG.l. Schematic outline afthe neutran-anticaincidence spectrometer,kept off the detectors by a shield of 'Li. The gamma rays are filtered bya few millimetres of lead to reduce the count rate from soft fission gammarays. The photons are detected in a Ge(Li) counter which is used as adouble-escape spectrometer. A lead shield of about 5 cm thickness betweensample material and neutron deteotor- prevents suppression of the capturegamma rays by coincident low-energy photons. A detailed analys is of them ma be found in Ref. [3] .GamID" r-ays arising from beta decay OL ,e lS ron .severe obstacle in the case of short measuring intervals. If all ha.lf-Livesbetween 350 msec and 1 h are included, the intensity ratio for the promptand delayed spectrum is about 2 : 1 at 4 MeV and about 10: 1 at 6 MeV.This was calculated in Ref. [3] from the experimental data reported inRefs [4~7].To demonstrate the feasibility of the neutron-anticoincidence spectro-meter, an experimental set-up ha s been installed at the reactor FR-2consisting of a Ge(Li) counter and a 44 crn d.iarn , X 60 cm plastic scintil-lator. The scintillator is provided with appr-opriate wells for the neutronbeam and the gamma detector. A schematic drawing of this instrumentis shown in Fig. 1. To obtain a maxirnum signal-to-background ratio thescintillator was bevelled on both ends.Delayed gamma rays occurring in the measured spectra will beidentified by means of a combined fast/ slow chopper system which hasbeen inatalled at the reactor to study the delayed radiation from fissionrer.A preliminary test spectrum obtained with the neutron-anticoincidericespectrometer and a spectrum taken in single mode are presented in Fig. 2.The energy interval ranges from about 3.4 to 6.4 MeV. The sample wasuranium oxide enriched 'to 20% in 235U. The spectra were measured usinga provisional electronic system. Onlya coarse adjustment of the variouscomponents had been performed. The speetra are therefore consideredas preliminary. Nevertheless, a clear structure already appears in theanticoincidence result. The suppression of the fission garnma rays cancertainly be improved further and probably the spectrum still contains\.-1,.,- 1 Ul(J:; 1N FISSIONABLE NUCLEI 53some contamination from delayed fission gamma rays and from neutroncapture in germanium caused by fission neutrons _ By placrng the deteetorin a- proper geometry relative to the plastic scintillator interference fromcaptur-e in germanium can be eliminated. In this case the scintillatorserves at the same time a s an anticoincidence shield for capture eventsoccurring in the detector.a)b)FIG.2. Preliminary results from a sarnple of uranturn enriched to 200/0 in 235 U: (a) speerrum taken inanticoincidence; (b) spectrum observed in single mode,54 MICHAELIS et al,It should be pointed out that 235 U is one of the most unfavourabletarget materials [3]. For other sample s the capture-to-fission ratiois much higher. It is, for instance, 0.358 for 239pu and about 0.39 for241 Pu.In summary, we can conclude that it will be possible to extend(n, /,)investigations also to fissionable nuclei.REFERENCES[1][2][3][4][ 5][6][7][ 8]MAIER-LEIBNITZ, H., ARMBRUSTER, P., SPECHT, H. J• , Physics and Chernistry of Fission(Proc, Symp. Salzburg, 1965)~, IAEA, Vienna (1965) 113.GREENWOOD, R.C. .: REED, J. H., IITRI - 1193 - 53 (Vols 1 and 2).MICHAELIS, W.,· Atomkernenergie (in press).MAIENSCHEIN, F.C., PEELE, R.W., ZOBEL, W•• LOVE, T.A., Int, Conf. peaceful Uses atom,Energy (Proc, Conf. Geneva,1958)15,UN, New York (1958) 366.RAU, F.E. W., Ann, Phys•• Paris 10 (1963) 252.PEELE, R.W., ZOBEL, W., LOVE,T. A., Rep. ORNL-2081 (1956)91.ZOBEL,W., LOVE, T.A.• Rep. ORNL-2081 (1956) 95.MICHAELIS, W., HaRSCH, F., LEUSCHNER, H•• WEITKAMP, C •• Trans. Am. nucl, sec. 11(1968) 663.Printed by the IAEA in Austria

Radiative neutron capture in fissionable nuclei

https://publikationen.bibliothek.kit.edu/270003353/3810259

Radiative neutron capture in fissionable nuclei

Abstract

Similar works

Full text

Available Versions

KITopen